KD2SL Repeater Update, January 2017

Yep, I’ve really let this website slide.  Life happens, with timing and intensity that are difficult to predict or control.

A *LOT* has happened in the past few years.  As time permits, I’ll try to bring you up to speed on all the repeater happenings.  When I last posted, the 10m repeater had just come online.  Since then, the 6m/10m system has been joined by three repeaters on 2m, two on 70cm, and one on 1.25m.  There are some interesting stories about those repeaters and how they came to be, and we’ll get to them soon.

For now, there is big news that needs to be front and center, and it relates to ALL of the repeaters.  Please read the following:

Jack Smith (W2QYT), Tony Hart (KC2VER) and Kevin Tubbs (KD2SL) are pleased to announce that we have reached an agreement with the Liverpool Amateur Repeater Club (a.k.a. LARC, W2CM), for LARC to become our sponsor for the following repeaters located on Sentinel Heights, between Syracuse and LaFayette:

  • 442.40 MHz (W2QYT), part of the statewide UHF system
  • 145.15 MHz (KD2SL), part of the Central New York linked 2m system
  • 53.67/29.64 MHz (KD2SL)
  • 146.67/444.00 MHz analog (KD2SL) and 145.31 MHz Fusion digital (KC2VER) linked system
  • 224.12 MHz (KD2SL)

These repeaters are located on transmitting towers owned by one of the nation’s largest broadcasting companies.  Many broadcasters do not allow amateur repeaters on their towers; thankfully this company takes a very friendly position toward amateur operation, and allows our repeaters on their towers at essentially no cost for tower space, inside equipment space and electricity.

However, the company has recently implemented new guidelines for amateur repeaters that requires them to be sponsored by a 501(c)(3) organization, rather than private individuals.  We reviewed our options:

  1. Form a new 501(c)(3) to become the owner and operator of the repeaters. This is much harder than it sounds, and would likely take much longer than the company was willing to wait.
  2. Enter into an alliance with an existing 501(c)(3), to become the sponsor or “umbrella” organization for the repeaters.
  3. Find new locations for the repeaters. This would likely require shutting down one or more of the repeaters, and splitting up the rest to different locations, with moderate to severe degradation in coverage area compared to what we now enjoy.
  4. Shut down some or all of the repeaters and call it a day. Nobody wants to see that happen, because repeaters on prime tower sites are always a strong asset to the amateur community.

We quickly concluded that option 2 was the best path forward, and reviewed the list of area ham clubs:

  • LARC – Has 501(c)(3) status
  • RAGS – Not a 501(c)(3)
  • QCWA – Not a 501(c)(3)

Discussions commenced with the LARC Board, which eventually resulted in the agreement that is being announced in this notice.

We anticipate that you have many questions, such as these:

Q. What exactly is the agreement? Does LARC now own and operate the repeaters?

A. There will be no ownership change. The KD2SL, KC2VER and W2QYT repeaters listed above will now operate under the umbrella of LARC, which means that LARC will sponsor our presence on the towers by signing the lease with the towers’ owner, and providing liability insurance coverage.  The ownership, maintenance and control of the repeaters does NOT change – it will continue to be handled by KD2SL, KC2VER and W2QYT, who are all LARC members.

Q. How can LARC sign the tower lease when they don’t actually own the repeaters?

A. We were completely up front with the towers’ owner about this, that the repeaters would be owned and operated by individual LARC members, rather than LARC corporately. The owner indicated that this did not conflict with the lease terms.

Q. What will change on the repeaters? Call signs?  Nets?

A. The biggest benefit to this arrangement is that the repeaters will remain in place, continuing to operate as they have been. The call signs will not change, and the nets will continue on the same schedule.  The only difference you’ll hear is occasional announcements thanking LARC for sponsoring our presence on the towers.

Q. Who pays for the lease and the insurance?

A. We three repeater owners will reimburse LARC for any and all costs that may be incurred as a result of this agreement. Due to the generosity of the towers’ owner, these costs are minimal.

Q. What about maintenance costs for the repeaters – radios, feedlines, antennas – that sort of thing?

A. The repeater owners will continue to be responsible for all of that. LARC’s role is to sponsor their presence on the tower sites.

Q. Do I have to join LARC if I want to use the KD2SL-KC2VER-W2QYT repeaters?

A. No. The KD2SL-KC2VER-W2QYT repeaters on the TV towers at Sentinel Heights continue to be open to all well-behaved hams, regardless of club affiliation.

If you have any questions, please contact any of the three repeater owners: KD2SL, KC2VER and W2QYT.  Please join us in thanking LARC for their sponsorship, which allows our repeaters to continue operating on the TV towers!  And thank you to everyone who uses our repeaters.

January 26, 2017

Kevin Tubbs, KD2SL – Jack Smith, W2QYT – Tony Hart, KC2VER

That pretty well sums it up!  Feel free to hit the comment link below, or write to me privately if you have any questions: kd2sl@yahoo.com

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Syracuse’s First 10 Meter Repeater

Another brief update:

Today, on a hunch, I connected the 10m receiver to the same antenna used for the 6m repeater. Amazingly, it actually works! So, we have a 10m repeater in Syracuse!

Feel free to give it a try, and let me know if you can get in. Also, whether or not you can get in, let me know what antenna, how much power, and where you are.

Remember, the 10m and 6m repeaters are permanently linked (crossband repeat), and you must have PL tone on transmit for either side. Program your radio carefully:

10m Repeater – Output: 29.64 MHz, PL 94.8 – Input: 29.54 MHz, PL 94.8
6m Repeater – Output: 53.67 MHz, PL 103.5 – Input: 52.67 MHz, PL 103.5

If your radio supports it, you can activate tone squelch on receive to keep the squelch from opening on noise.

Join us each Wednesday at 7:00 pm for Tim Colson’s (N2VZD) Central New York Swap and Information net, now a tri-band net!

73 – KD2SL

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10m Repeater Now On…Kinda…Sorta…

A quick note to let you know that I’ve added “experimental” 10 meter RX and TX to the repeater. The RX is VERY deaf, so don’t even bother trying to hit it.

However, I would be interested in receiving your reception reports from the 10m repeater TX, which is 29.640 MHz. It should be transmitting anytime 53.670 is transmitting.

If anyone knows of a good spot for a 10m receive antenna, that can be linked on UHF to the repeater site at Sentinel Heights, please let me know.

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Repeater Version 2.0

Repeater version 2.0, on its way from the car to the transmitter building. That thing weighs at least 125 lbs – good old ‘merican engineering!

New and different?  Yes!  Better?  In some ways.

Back in May of this year, when the 53.670 MHz repeater went on the air, I received an e-mail from a very wise person.  He said, “Congratulations!  But, this is just the beginning.  You will NEVER be finished working on the repeater.”  So very true.

Not long after repeater version 1.0 went on the air, I received a donation of yet another VHF low band Mastr II.  My thought was, “I can bring this home, take my time studying and converting it to 6 meters, and have a spare!”  Well, as most hams can attest, little projects mushroom into bigger projects, consuming way more time and money than you expected.

Now that repeater version 2.0 is on the air, I’ll have a little more spare time to document what happened.  Stay tuned for more frequent posts in the coming weeks, and I’ll fill you in on what’s under the hood.

73 – KD2SL

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Yet Another Antenna Change

UPDATE, December 13, 2012: The antenna configuration is back to “normal,” (and has been since late September, actually) with both TX and RX from the top antenna.

I’ve changed the antenna configuration again.  I think we got all the data we needed from yesterday’s experiment.  To review, on Thursday morning I installed a 1/4 wave ground plane near the base of the tower, about 800 feet below the old channel 3 antenna.  I fed the receiver from this new antenna, but left transmit on the top antenna.  As expected, this degraded the repeater’s ability to hear, primarily because of the difference in height.  Repeater transmit was not affected.

Now, as of 1:30pm Friday (Sept. 21), it has been reversed.  We’re receiving from the top antenna (horizontal polarity), and transmitting from the bottom (vertical polarity).  This will degrade your ability to hear the repeater, especially in fringe areas, but the repeater’s receiver should be working better than ever (no cracklies or desense from intermod).

Thanks to EVERYONE, you have all been really great sports, willing to play numerous rounds of “can you hear me now?”!  I’m very interested to know how well you can hear the repeater in this current arrangement.

73 – KD2SL

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Antenna Change – Reports Wanted

As of about 10am today (Thursday Sept. 20), we are now transmitting on the top antenna, but receiving on a new antenna – a vertical (ground plane) mounted at the base of the tower.  I am very interested to hear your observations.  Your reception of the repeater should be the same, but there will definitely be some differences on the repeater’s receive side.  It is now vertical (big improvement), but it is 800 feet lower (big loss).  Did we gain more than we lost?  Also, the cracklies should be completely gone.  Yet one more positive factor is that we probably have less site noise at this antenna location (i.e., we don’t have kilowatts of UHF being crammed down our throat from the TV broadcast antenna).

So, it is a big unknown.  An experiment.  In a few days, we’ll flip flop and receive from the top, transmit from the bottom.

73 – KD2SL.

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Catching Up

We’re WAYYYYYYY overdue for an update!  It has been a very busy summer; I’ve been doing my best to enjoy it, and that has meant less time for writing blog posts.  Maybe it also means less time for READING them!  At any rate, I’ll get back to the duplexer story in the next post.  But for today, let’s take a moment and review where we stand:


I’m very happy that so many have used the repeater, and have been experimenting with antennas and rigs.  We have maybe a dozen regular users, another dozen who show up every once in a while, and an unknown number that listen regularly but don’t transmit.  There is always room for more, please join in!  Everyone is welcome.  The repeater doesn’t “belong” to any one group or person; it is there for all hams to use and enjoy. 


I have often been asked whether we will host a net on this repeater.  I’m open to the idea, but I don’t have any plans to start a new net.  Rather, I would like to simulcast (link) to an existing net, such as the Wednesday night 7:30pm LARC net on 146.91.  The repeater’s hardware doesn’t provide a way to do this, but an upgrade is planned soon.  When we’re ready to turn on a link to that net (or others), readers of this blog will be the first to know!


We’re getting a feel for the repeater’s range.  By all accounts, the transmit range of the repeater is VERY good, even when receiving cross-polarity.  Adirondacks.  Catskills.  Watertown.  Binghamton.  Rochester.  Ontario, Canada.  Northern Pennsylvania.  Western NY hills.  Vermont.  Some of the signal reports from these areas run anywhere from S1 to S9+.  However, one common thread is that some people (especially more distant stations) can hear it, but not bring it up.  We would wish that anyone who can hear it could work it, so it puts our minds to thinking: Why can’t they?

I can think of a few reasons why this might happen:

  • Not enough power.  If you’re on the fringe of the repeater’s reach, generally speaking you would need about as much power as the repeater (approximately 100 watts).
  • Poor antenna performance.  Situations can arise where an antenna performs better on receive than it does on transmit.
  • CTCSS tone.  Some hams have had difficulty getting their radio programmed properly for transmitting the PL tone, so we have to consider that as one possibility.  Also, one ham says the repeater’s receiver is too sensitive to tone level on the received signal, and that he had to increase his rig’s tone deviation before he could open the repeater’s squelch.  So far, he is the only one to contact me with this report.
  • 4 – Polarity.  This point gets a LOT of discussion.  Yes, this is a horizontally polarized repeater, and using vertical antennas will reduce your range.  But, a polarity mismatch theoretically reduces your range in BOTH directions equally.  When it comes to hearing the repeater but not being able to work it, I don’t think this is a significant factor.
  • Repeater sensitivity.  This may be the biggest reason.  And I’m not talking about simple bench sensitiviy.  It is a sad fact of two-way radio life that repeater sites are usually very noisy.  This would be a great topic for a future, in-depth post, because it is one of the biggest factors affecting repeater performance.  In short, the collection of broadcast stations typically found at prime hilltop locations produce some amount of wideband noise in addition to their intended signal.  It is kind of like the hiss you hear if you put your ear right up to a speaker on your home theater system.  Such noise from high power transmitters is very low in comparison to their main output signal, but it can, and does impact the ability of a repeater receiver to hear weak signals.  At our location, I’ve measured this RF hash at about 15 dBm above the background noise of the test instrument, or -110 dBm, which is around .7 uV.  Chances are that your receiver isn’t seeing anywhere near that amount of noise, so you have a much better effective sensitivity than the repeater.  In addition to site noise, tests have confirmed that we’re getting an additional 6 dBm of desense from intermod involving the transmitter.  If all of these numbers (and my math) are correct, that’s around 20 dBm (in round figures) of desense.  20 dBm is the difference between 100 watts and 1 watt.  Look at it this way: If you can now work the repeater with 100 watts from a distant location, you could do it with just 1 watt if we could magically remove all the desense.  Think of what THAT could do for the repeater’s range!

Combine some mix of all of these things, and you can easily see why we are getting some reports of an alligator repeater (all mouth and no ears).  The GOOD news is, some of these things can be fixed.  That’s a topic for future posts.


July 17 brought us a fabulous gift in the form of a strong sporadic E skip opening on 6 meters.  I heard reports of great DX on SSB and FM simplex.  If you turned off your rig’s tone encode (to avoid bringing up K2INH 53.05 in Auburn) and kerchunked 53.05, you could hear at least three repeaters coming back!

I worked Kentucky on a repeater on 53.11 (not sure where the repeater actually was).  And, several locals had a rather lengthy QSO on 53.67 with a ham from Daytona Beach, Florida!  It was very exciting to see what 6 meters can do, and why they call it the “Magic Band!”

Aside from that great day, we’ve heard various other reports during this repeater’s first four months.  A ham from Windsor, Ontario (near Detroit) has reported hearing us on several occasions.  One local reported hearing a brief signal on the repeater from Hudson Bay, Canada.  We all look forward to the next big opening! 


GE Mastr II Low Band RadioThanks to a generous local ham, I have another complete GE Mastr II repeater!  At first I thought it would just go in to storage and be used for parts, but before long I decided to completely refurbish it to become the “new” KD2SL repeater.  We’ll give it a spiffy new controller that will allow for linking and/or a remote base.  The receiver and transmitter will be thoroughly rehabbed to squeeze out every last ounce of performance. 

The rush to get the original repeater on the air back in May did not allow the luxury of time to make these performance modifications, so I’m enjoying the opportunity to do them now, and learn more about the Mastr II in the process.  A low band Mastr II typically is designed to cover approximately 6 MHz of spectrum.  If you want to operate below or above the original design range, it might work, or it might not.  Many of the tuning adjustments in the receiver and exciter can’t tune far enough (i.e., the ferrite slugs won’t go far enough without falling out of the coil) for proper setup in the 6m band unless certain components (capacitors, mostly) are changed.  Again, I’ll have more about this in a future post.

Another important upgrade is the new repeater controller I alluded to earlier.  This will open up various linking opportunities, easier control of various features, and provide more options for identifications and announcements.

Is EchoLink or AllStar in the repeater’s future?  Maybe.


I have obtained a couple of commercial low band antennas – a dipole, and a ground plane.  I intend to install one or both of them at the repeater site, and conduct some experiments to compare their performance to the TV broadcast antenna.  How much difference does antenna polarity make?  How high above ground does a 6m antenna need to be?  Will a different antenna, or split TX-RX antennas provide better repeater performance?  Less crackle?  We’ll experiment and find out!


Thanks again to all who have been taken time to use the repeater!  More posts are coming, with information about future upgrade plans, as well as the conclusion of the story of how the current repeater was built.  Until next time, 73 and I hope to hear you on the repeater!

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Duplexer Construction – Part 2 (The Prototype)


Prototype cavity, just before resonator stub installation

The prototype cavity, just before installation of the resonator stub.

During the late part of winter 2012, I spent a great deal of time reading up on duplexer theory and construction.  I really wanted to understand what I was doing, not just blindly follow a recipe to build something.  The more I read, the more I believed it would be possible to build a properly functioning unit using parts from the former TV diplexer and sideband filter.  If the telescoping resonator stub sections could tune low enough, or be modified to tune low enough, then we should be in good shape.

By this time, I had brought John (WB2DVE) on board to help with machining tasks needed for duplexer construction.  His first assignment had three parts: 1) Coupling loops – create discs with a BNC and an N connector mounted on them; 2) Top plate – holes with shoulders to receive the coupling loop discs; 3) Inside partition – cut two pieces to length to make one complete partition.  A few holes would need to be drilled in the outside panels to match the nuts on the partition; I decided to do this on-site with a handheld drill. 

The prototype coupling loop.

The prototype coupling loop, made with thick strap scavenged from the TV diplexer.

I also used some strap pieces removed from the diplexer to create coupling loops.  The shape and dimensions of the coupling loops were copied directly from this web page, by Gary (NZ5V).  John provided the BNC connectors, I recovered the N connectors from obsolete equipment in the “junk pile.”  Two identical coupling loops were built, and two holes were cut in the top plate.  John cut an extra, blank disc to fill one of the holes so that I could test both bandpass and bandpass-bandreject (BpBr) configurations.

KD2SL Duplexer

Closeup of the holes in the top plate, which will receive the coupling loops.

Due to the size of the 6-cavity cabinet, all of the assembly and testing of the prototype had to take place at the transmitter site, which is a 20 minute drive from the office.  As soon as I had all of the machined and cut pieces from John, I eagerly arranged a long lunch break to assemble them into the prototype cavity.  John’s work was flawless, and everything fit together perfectly.  You will recall from an earlier post that one of the resonator stubs was longer than the others; I made sure to use that one for the prototype, so as to provide the best chance for success at the lower frequency.

KD2SL Duplexer

With the cavity tuned to 53.67 MHz, there was only 1/2″ of adjustment rod left!

Testing the resonant frequency of the cavity is pretty straightforward: connect a signal generator to one coupling loop, a spectrum analyzer to the other, and see what comes out!  The peak in cavity response is easy to spot.  I kept pushing the tuning rod lower and lower, and reached resonance at 53.67 MHz (our transmit frequency) with less than an inch of the adjusting rod to spare!  It was not possible to reach 52.67 MHz (the repeater’s receive frequency), but at least we now know how long a resonator stub needs to be for resonance at our repeater’s frequencies, and that it will indeed fit (barely) within the 51″ height of the cabinet.

KD2SL Duplexer

It doesn’t look so “fat” in this picture, but duplexer cavities normally use skinnier resonator stubs.

Why does it fit, when the calculation of a 1/4 wavelength at our frequency comes out a little longer than 51″?  Because the resonator stub is fat.  This same principle applies to antenna design; for example, with a yagi (beam) or dipole, making the elements fatter decreases their resonant frequency, meaning that you end up making them shorter to compensate.  It also makes them a little more broadbanded, which is usually great for antennas, but it reduces the “Q” of a resonant cavity.  We could have achieved higher “Q” and better notch depth with a skinnier resonator stub, but at our frequency, it would have been too long to fit in the cabinet.  There are always tradeoffs, it seems.  Fortunately, this is one we can live with.

Resonance alone isn’t the whole story.  How is the “Q”?  What the heck is “Q” anyway?  “Q” basically stands for Quality, and with regard to filters or other RF components, it refers to how sharp the response is.  Think of how an antenna works.  If it is broadband and covers a wide frequency range, it has a low Q.  If it has a narrow frequency range, it has a higher Q.  For duplexer cavities, we want a very high Q, because the distance between a cavity’s notch and passband is very small (only 1 MHz in our case).  The high Q means that a single cavity can attenuate 52.67 MHz by 40 dB, yet 53.67 MHz escapes virtually unscathed.

I’m happy to say that the Q of the prototype cavity was pretty good for duplexer use.  I didn’t think to make notes or take pictures of the spectrum analyzer while testing the prototype, so you’ll have to take my word for it.  In the bandpass configuration, insertion loss (attenuation at the pass frequency) was around .4 or .5 dB with the coupling loops adjusted for maximum coupling, and the response rolled off quite sharply – something like 15 dB or more at 1 MHz out from the center frequency.

Reasonably high Q, and the resonator stubs will tune low enough while still fitting in the cabinet.  Life is good!  Time to go full steam ahead with construction, and that’s where we’ll pick up the story next time.

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Duplexer Construction – Part 1 (The Components)

Fully stripped filter cabinets

The bare cabinets. Just out of view is a BIG pile of hardware!

Alright, it has been exciting to launch the repeater, and I’ve been focused on that the past couple of weeks – but let’s get back to how it got here!  When we last talked about the duplexer, I was deciding whether to build in the 6-cavity cabinet, or the 4-cavity cabinet.  You know by now that I decided on the 4-cavity cabinet, but why?

Advantages for the 6-cavity design are that I could include a bandpass filter for receive, which may or may not be necessary for protection of the receiver, due to nearby broadcast transmitters.  Advantages for the 4-cavity design would be smaller size and easier transportation (the whole thing fits in my wife’s Honda CR-V), and the inside partitions were nearly complete.  I was leaning toward the 4-cavity cabinet, but decided that the very first thing I needed to do was to build a prototype cavity in the 6-cavity box first, to see how it performed.  That way, if the duplexer is built in the 4-cavity box, I’ll still have the prototype to experiment with.

Inside the filter

Inside the bandpass filter. Click on the image for a super Hi-Res version.

Take another look at the 6-cavity cabinet (before it was fully desecrated!)  Notice that the left-most resonator has a longer bottom section.  This is because it needs to tune to a lower frequency, since it masks (or shunts) the low side of the passband.  For TV channel 3, this means about 60 MHz.  Assuming that the same design cabinet would be used for TV channel 2, which covers 54 to 60 MHz, I expected that the longest resonator could probably be tuned below 54 MHz.  The other resonators were all a few inches shorter and probably couldn’t be tuned that low.  LOTS of assumptions.  I needed answers!  “How low would it go” was the most important question that the prototype would answer.

Silver-plated telescoping resonator stubs.

Silver-plated, telescoping resonator stubs. NOT your average hardware store item!

Taking apart the diplexer and filter cabinets yielded a very nice pile of high quality metals; most of the cabinet was plated aluminum (not sure with what), and the resonators were silver-plated copper pipe with silver-plated finger stock in the slip joints.  Fancy stuff!  Building high-quality, stable, telescoping resonator stubs is the hardest part of a homebrew duplexer, so it is nice to have that work done for us!  Overall, it appeared that there was enough material that could be cut and reassembled to make duplexer cavities, but there wasn’t much room for mistakes or do-overs.  I did NOT want to buy any additional metal!  It was important to get it right the first time, important to do the research.

Generally speaking, there are three major components to a resonant cavity:

  1. The enclosure
  2. The resonator stub
  3. The coupling loop(s)

The Enclosure

The enclosures most often used for resonant cavities are usually round pipes, but they can also be square like the ones we’re working with.  Ours happen to be about 8″ square, and about 4 ft. 3 in. tall.  Eight inches square is perfectly fine, but only 4 ft. 3 in. tall?  That’s Aluminum partscutting it awfully close for a quarter wave at 6 meters.  In space (rather than in wire or metal), a quarter wave at 52.67 MHz is approximately 246 / 52.67 = 4.67 ft., a few inches LONGER than our enclosure.  We’ll have to hope that our prototype cavity has a low enough velocity factor to make it fit!

Notice that if we build the prototype at the left end of this enclosure, we already have a top (the 10mm thick plate leaning against the side), bottom, front (removed for the picture), back, and one side; we just need to build a bigger partition to close it off on the inside.  No problem, we’ll just borrow some partition material from elsewhere in the cabinet, and cut it to fill out the partition to reach all the way from the top to the bottom.  That will give us a fully enclosed box that measures about 8″ x 8″ x 51″.  If it works OK, we can build more cavities using a similar method.

The Resonator Stub

A resonant cavity is just a 1/4 wave section of coax – very LARGE diameter coax – open at one end and shorted at the other.  The length of the center conductor determines the resonant frequency.  You can’t accurately measure the center conductor and cut it to length ahead of time; you need some way to fine tune it by adjusting the length after it is in Telescoping resonator stubthe enclosure.  Usually some sort of telescoping section is used to accomplish this, and the resonator stubs from the old combiner and filter cabinets do just that.  We have TEN of them to work with, but we only need 4 (or maybe 5 or 6 if we add bandpass cavities), so maybe we can somehow combine them to make them longer, if necessary.

Something worth noting about these resonator stubs is the diameter, about 2-3/4″ for the top section, and about 4″ for the bottom.  That is somewhat larger than ideal for an 8″ square cavity.  The more air space you have between the cavity walls and the center stub, the better performance you get.  Commercial designs for duplexer cavities usually have thinner stubs, which results in a longer cavity, larger cavity volume, and higher “Q” (sharper tuning).  However, you can’t change any ONE thing in a cavity filter without affecting some OTHER thing.  In this case, that’s OK, because having a fatter stub means that it will be shorter, and our cabinet isn’t very tall.  A skinnier stub would probably be too long to fit, so having a little lower “Q” is a reasonable tradeoff for being able to make it fit!

The Coupling Loop

The diplexer and filter cabinets did not have coupling loops that were useful for our application.  We will have to fabricate our own.  Coupling loops are really just half of a transformer; they magnetically couple the RF into the center resonator stub (the other

The prototype coupling loop.

The prototype coupling loop, made with thick strap scavenged from the TV diplexer.

half of the transformer).  The basic design is shown in the photo; one end of the loop connects to the coax center conductor, the other end connects to a capacitor (for a reject cavity), or to ground (for a bandpass cavity).  Using a connector for both ends of the loop gives you the flexibility to go either way.  Different types of capacitors may be used, and can be mounted either inside or outside the cavity (we’re using a connector, so the capacitor will be outside).

As far as mounting the coupling loop to the top plate, the most common design uses a disc that fits into a hole, and is screwed in place.  To get the desired filter performance, you adjust the amount of coupling by loosening the screws and rotating the disc.  This alters the alignment of the magnetic field inside the cavity, and therefore alters the filter performance.  The top plate of our cabinet is 10mm thick aluminum, so we’ll make some discs out of material that was recovered from disassembling the cabinets, and make some holes in the top plate to receive the discs.

OK, we have a basic plan to cover the basic elements of cavity design.  I’m really itching to get a prototype assembled, and answer the BIG questions: How long does the resonator stub need to be for 53.67 (TX) and 52.67 MHz (RX), and will it fit in our cabinet?  And, how well will it work (how high will the “Q” be?)  In our next installment, we’ll see more particulars of how the prototype cavity was put together, and see how it performed.

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6m HT


The “new” official KD2SL office monitor rig. (A convenient excuse to try out the WordPress app for my phone.)

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